Technical Field
The present invention relates to power systems, and more particularly to synchronization control for reconnecting a microgrid to a main grid after islanding.
Description of the Related Art
With increasing penetration of distributed energy resources, the microgrid concept is used to enhance the reliability of power systems by integrating distributed generators (DGs) with a load in a local distribution network. A microgrid can operate under both grid-tied mode and islanded mode. Under the grid-tied mode, it is connected to a medium voltage network, either absorbing power from or injecting power into the main grid. In the case of an emergency, the microgrid is disconnected from the main grid and starts working autonomously, in a similar way to physical islands. There are many technical difficulties in microgrid islanding and perhaps the most challenging one is the out-of-phase reclosing. After islanding, a microgrid will accelerate or decelerate due to power imbalance, losing synchronization with the main grid. When an event (scheduled maintenance/fault/etc.) that triggers islanding disappears, the circuit breaker (CB) or re-closer will try to connect the microgrid back to the main grid. At the moment of reconnection, lack of synchronization can lead to severe consequences. For example, if the difference in voltage phase angles across the circuit breaker reaches 60 degrees, voltage across the circuit breaker will be around 1 pu. If reconnection occurs at this time instant, the voltage difference will induce a huge inrush of current flowing between the main grid and the distributed generators, causing system oscillations or potential damage to equipment. A larger inrush current can be induced if the two systems are completely out of phase. To achieve a smooth and successful reconnection, the voltage at the PCC (point of common coupling) on the microgrid side must be synchronized with that on the utility grid side.